Fully optical analog to digital converters with complementary outputs

ABSTRACT

A fully optical converter for converting an analog signal into a digital signal includes an amplitude to optical wavelength converter for converting the analog signal into an optical signal which varies in wavelength in accordance with an amplitude of the analog signal, a splitter for applying the optical signal over a desired number of light paths, an interferometer connected to each of the light paths, unequal path lengths in each leg of the interferometers to allow optical interference to deliver a complementary sinusoidal transfer function to the optical signal to generate two complimentary output signals and a dual detector connected to each of the interferometers for generating a digital bit in response to the two complimentary output signals, wherein each of the digital bits together form a parallel digital word. The fully optically based converter is automatic and independent of interactive techniques, thus providing for an expedited conversion rate.

BACKGROUND OF THE INVENTION

The present invention relates generally to analog to digital converters,and more particularly to all optical analog to digital converters.

It is often desirable to convert an analog amplitude varying signal to adigital set of values which corresponds to various voltages in theanalog waveform to generate a corresponding digital signal. Conventionalapproaches generally rely on iterative and/or comparative techniques fordetermining a digital signal based on an analog waveform voltage. Inparticular, a common conventional approach compares the actual voltageof the analog amplitude varying signal to a comparison voltage which isgenerated from a digital word. Various digital words are utilized tocreate comparison voltages which are then rapidly compared to the actualvoltage to determine whether the comparison voltages are greater or lessthan, in an instant of time, the analog amplitude varying signal.Through a continuous iterative and/or comparative process, a digitalword which corresponds to the actual voltage of the analog amplitudevarying signal is generated. The digital word is recorded for thatinstant of time and the same iterative and/or comparative process isrepeated for subsequent instants of time corresponding to the analogsignal. This conventional method suffers from various shortcomings,including but not limited to, errors and time inefficiency due to theiterative process.

What is needed therefore is an apparatus and method for converting ananalog signal into a digital signal which is automatic, accurate, timeefficient and does not rely on iterative techniques.

SUMMARY OF THE INVENTION

The preceding and other shortcomings of the prior art are addressed andovercome by the present invention which provides, in a first aspect, anapparatus for converting an analog signal into a digital signal,including a converter which converts the analog signal into an opticalsignal which varies in wavelength in accordance with an amplitude of theanalog signal, a splitter which applies the optical signal over apreselected number of light paths, an interferometer connected to eachof the light paths which applies a sinusoidal transfer function to theoptical signal to generate two complimentary output signals; and, adetector connected to each of the interferometers which generates adigital bit in response to the two complimentary output signals, whereineach of the digital bits are combined to form a parallel digital word.

In another aspect, the present invention provides a method forconverting an analog signal into a digital signal, including the stepsof converting the analog signal into an optical signal which varies inwavelength in accordance with an amplitude of the analog signal,splitting the optical signal over a preselected number of light paths,applying a sinusoidal transfer function to the optical signal over eachof the light paths to generate two complimentary output signals andgenerating a digital bit in response to the two complimentary outputsignals, wherein each of the digital bits are combined to form aparallel digital word.

The foregoing and additional features and advantages of this inventionwill become apparent from the detailed description and accompanyingdrawing figures below. In the figures and the written description,numerals indicate the various features of the invention, like numeralsreferring to like features throughout both the drawing figures and thewritten description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a fully optical analog to digitalconverter in accordance with the present invention; and

FIG. 2 is a detailed diagram of the detector circuit illustrated in FIG.1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As is illustrated in FIG. 1, the present invention provides a fullyoptical analog to digital converter 10 which is automatic andindependent of iterative techniques, thus providing for an expeditedconversion rate. In particular, the converter 10 includes an amplitudeto optical wavelength converter 12, optical splitter 14, interferometers16 and detectors 18. The fully optical analog to digital converter 10initially converts an analog signal 20, such as a signal in the radiofrequency (RF) range, into an optical signal 22 which varies inwavelength in accordance with the analog signal 20. The amplitude of theoptical signal 22 remains constant as the wavelength is varied over arange which corresponds to the analog amplitude of the analog signal 20.In accordance with the present invention, the wavelength variation onthe optical signal 22 is utilized to generate a corresponding paralleldigital word 24.

In particular, referring to FIG. 1, the analog signal 20 is applied tothe amplitude to optical wavelength converter 12 for converting theanalog signal 20 into an optical signal 22 which varies in wavelength inaccordance with the amplitude of the analog signal 20. The amplitude tooptical wavelength converter 12, preferably one or more laser diodes,has an output which is a function in wavelength of how the converter 12is driven. For example, a laser diode which is driven by a largeramplitude signal will generally have a longer output wavelength than alaser diode driven by a smaller amplitude signal. The present inventionis not limited to the use of laser diodes for converting the analogsignal 20 into an optical signal, but rather any device including abroadband light source having a large bandwidth for optical gain can beutilized in conjunction with an optical filter that tunes with thesignal amplitude. Such devices include, but are not limited to, fiberlasers, optical fiber amplifiers, and other solid state amplifiersystems.

The wavelength varying optical signal 22 generated by the converter 12is applied to the splitter 14 which splits the optical signal 22 into adesired number of light paths 26. Each path 26 is utilized to create adigital bit 28 in the final parallel digital word 24. The number ofpaths 26 the wavelength varying optical signal 22 is split into isdependent on the desired level of resolution. For example, asillustrated in FIG. 1, for 8-bit resolution (256 signal levels), thewavelength varying optical signal 22 is split into eight light paths 26.The splitter 14 is preferably an active multimode signal splitter, suchas the splitter disclosed in U.S. patent application Ser. No.08/866,656, filed May 30, 1997, entitled "Active Multimode OpticalSignal Splitter", assigned to the same Assignee as the present inventionand incorporated herein for reference. The splitter 14, in addition tosplitting the optical signal 22, maintains the intensity of each opticalsignal along each light path 26 approximately the same, thus allowingthe optical signal 22 to be split into a greater number of equal lightpaths 26 without loss in light intensity. The optical splitter 14 couldalternatively be a conventional or newly developed splitter, such as afiber optic star coupler which is manufactured from a group of opticalfibers which have their cladding layer removed prior to being twistedtogether. This allows the light in one fiber to evanescently coupleequally into all the other fibers thereby allowing the wavelengthvarying optical signal 22 to be split.

As is illustrated in FIG. 1, each of the split light paths 26 is appliedto an interferometer 16, preferably a Mach-Zender interferometer, whichsplits the light from each path 26 into two split out interferometerpaths 30 and 32 and then combines the two paths 30 and 32interferometrically to generate two complimentary output signals 40 and42. The two path lengths 30 and 32 of each interferometer 16 are unequalin length by an amount corresponding to the weighting factor requiredfor a particular bit that the split light path is to represent in thefinal parallel digital word 24. In particular, the light 26 is splitinto the two unequal path lengths 30 and 32 of the interferometer 16 sothat when the two paths 30 and 32 are combined, optical interferencedirects the light proportionally into the two complimentary outputsignals 40 and 42 according to a sinusoidal transfer function dependingon the wavelength of light. The difference in the length of the twopaths 30 and 32 is determined by the optical path length differencerequired to make the desired optical interference cycle through thecomplementary sinusoidal partition of the light between legs 30 and 32the same number of times as the particular bit changes in the paralleldigital word 28. Each interferometer 16 has a unique path lengthdifference corresponding to each digital bit. The longer path 30 in eachMach-Zender interferometer 16 is generally different from the othersenabling the short path 32 to be the same length for allinterferometers.

In operation, as the wavelength of light is swept through the range ofwavelengths corresponding to the analog signal amplitude, the output ofeach interferometer 16 is a complementary sinusoidal variation in theintensity partitioned between the two output signals 40 and 42. Theinterferometer 16 for the most significant bit delivers just one cycleof variation in intensity. The interferometer 16 for the next mostsignificant bit experiences two complete cycles. The next mostsignificant bit experiences four cycles, the next experiences eightcycles, and the pattern continues to increase as a power of two forsuccessively less significant digital bits. The most significant bit(MSB) 34 will only go through one complementary sinusoidal cycle changefor the entire analog domain. For 8-bit resolution, the leastsignificant bit (LSB) 36, the eighth bit, will go through one hundredand twenty eight complementary sinusoidal cycles of variation inintensity for the entire analog domain. The interferometer 16 may be amonolithic frequency division multiplexer (FDM) manufactured by PhotonicIntegration and Research, Inc. of Columbus, Ohio, under a typical modelnumber like FDM-10G-1.5, which corresponds to an unbalancedinterferometer that delivers one cycle of intensity variation when thelight wavelength at 1.5 um is changed by 0.08 nm (a frequency change of10 GHz). Although the interferometers 16 are illustrated in FIG. 1 asseparate units, they are not limited to such a configuration but rathermay be configured monolithically on the same wafer device. Theinterferometers may also be made from fiber optic cable and fiber opticcouplers through the use of either polarization maintaining fiber orpolarization controllers.

The output light distribution in the two output legs 40 and 42 isdetermined by the particular wavelength of light that is initiallyapplied to the interferometer 16. The wavelength of light establishesthe particular interference phase state with which the light from thetwo unequal paths 30 and 32 interferes when they are combined at theoutput of the interferometer 16. This phase state determines how thelight becomes partitioned into the two output legs 40 and 42. The lengthof the optical delay in path 30 relative to path 32 determines thewavelength range over which the light must be varied to establish thesame phase state corresponding to a full complementary sinusoidalintensity variation that will lead to the same partition of light in thetwo output legs 40 and 42. To achieve this sinusoidal intensityvariation, the split ratio dividing the light into paths 30 and 32, thecombining ratio at the output of the interferometer 16, and the splitratio into output legs 40 and 42 all need to be approximately 50:50. Inaccordance with this complementary sinusoidal intensity variation, thereis a particular phase state that leads to a maximum intensity in leg 40and a minimum intensity in leg 42 followed by a continuum of phasestates that smoothly reverses the intensities in each leg. Continuedchange of the phase state via a change in the wavelength will restorethe original maximum intensity in leg 40 and the minimum intensity inleg 42 to constitute one full complementary sinusoidal intensityvariation cycle. In operation, for the most significant bit (MSB) 34,the smallest to largest wavelength change corresponding to the entireanalog domain is swept through the interferometer 16 creating just onecomplementary sinusoidal intensity variation. For 8-bit resolution, theleast significant bit (LSB) 36 will go through one hundred and twentyeight complementary sinusoidal intensity variations for the entireanalog domain.

Referring to FIG. 2, the two complimentary output signals 40 and 42 fromeach interferometer 16 present a complementary sinusoidal variation suchthat the output signals 40 and 42, once applied to the dual detector 18,are directly converted to an electronic digital bit 28. In particular,the two complementary optical output signals 40 and 42 of theinterferometer 16 are applied to the dual detector 18, preferably abalanced detector including photodiodes 44 and 46 and a limitingamplifier 48. The limiting electronic amplifier 48 converts the signalto a digital "0" or "1" depending on the optical intensity delivered bythe interferometer 16. Thus, for the entire first part of thecomplementary sinusoidal variation cycle, where more light is deliveredfrom one of the two complementary outputs 40 and 42 of theinterferometer 16, the electrical output 28 would correspond to a "1".For the entire second portion of the cycle, where there is more light inthe other complementary output of the interferometer 16, the output 28would correspond to a "0".

As is illustrated in FIG. 2, the photodiodes 44 and 46 are connected inseries in a balanced configuration. In particular, the anode 52 of onephotodiode 44 is connected to the cathode 50 of the other photodiode 46.When light is applied to the photodiode 44, current is conducted fromthe input of the limiting electronic amplifier 48 in a direction asshown by the arrow associated with the photodiode 44. When light isapplied to the other photodiode 46, current is conducted in an oppositedirection into the input of the limiting electronic amplifier 48 asshown by the arrow associated with the photodiode 46.

Referring to FIGS. 1 and 2, when applied to the photodiode 44, the firstcomplimentary output signal 40 creates a current having a magnitudeproportional to the intensity of the light in the first complimentaryoutput signal 40. Similarly, when applied to the photodiode 46, thesecond complimentary output signal 42 creates a current having amagnitude proportional to the intensity of the light in secondcomplimentary output signal 42. If the resulting current is applied tothe amplifier 48 in a first direction, then the first complimentaryoutput signals 40 is greater in intensity. Similarly, if the resultingcurrent is applied to the limiting electronic amplifier 48 in a seconddirection, then the second complimentary output signals 42 is greater inintensity. In this way, it can be determined which of the two legs 40 or42 is greater in intensity. Thus, in accordance with the presentinvention, it is unnecessary to know the absolute level of light orcompare the absolute level of light to any other value to determinewhether the absolute level of light is above or below a threshold value.In addition, this relative comparison of the light intensities in thetwo complimentary output signals 40 and 42 enables the use of converters12 which vary in output intensity with changes in wavelength, as is thecase with laser diodes.

In the present invention, by determining when the light swings frombeing m ore intense in one complimentary output signal 40 than in theother complimentary output signal 42, the transition between a "0" and"1" state for a bit can be determined. The transition occurs at aparticular distinct point which is a function of a particular wavelengthof light which is sent through the interferometer 16 and is notinfluenced by the intensity level delivered to interferometer 16.Through the use of a limiting electronic amplifier 48, the output signal28 can be made to achieve commonly used digital voltage levels for "0"and "1" bits.

The present invention is not limited to the detector described orillustrated herein. The present invention may also be utilized withother detectors, including but not limited to the two complimentaryoutput signals 40 and 42 of the interferometer 16 being connected to aphotodiode (not shown) whose output is applied individually to atransimpedance amplifier (not shown) that is uniquely associated witheach photodiode. The output of one transimpedance amplifier is thenelectrically inverted and resistively summed with the output of theother transimpedance amplifier to create a resulting signal that is thensent to a limiting amplifier to create electronic digital "1" and "0"bits for the output signal 28. The detector configuration describedherein is preferable for operation at higher speeds where the photodiodecapacitance may effect the output because the capacitance becomesdoubled when photodiodes are connected together as anode to cathode. Inaddition, the individual connection of each photodiode to its ownelectronic amplifier allows the use of resistively terminatedphotodiodes commercially available from manufactures such as Lasertronas QDMH1-055 where a 50 ohm resistor is packaged directly across thephotodiode for impedance matching to 50 ohms. Thus, in accordance withan advantage of the present invention, every analog voltage is initiallyconverted to a particular wavelength utilizing a one-to-onecorrespondence. An optical light beam 22 having a particular wavelengthis generated and transmitted through an interferometer 16, which througha conversion process outputs a parallel digital word 24, without relyingon a trial and error process.

In operation, in a typical configuration, a laser diode 12 modulable inwavelength by 0.08 nm via variation of the drive current is directlymodulated by an analog RF signal 20 to create a wavelength varyingoptical signal 22. For a typical distributed feedback (DFB) laser diode,a 9 ma current variation to the drive of the laser diode 12 operating at1.55 um creates a wavelength variation of 0.08 nm that will deliver asingle complementary sinusoidal intensity variation cycle in asingle-mode fiber Mach-Zender interferometer 16 that has a pathdifference of 2 cm assuming an optical fiber refractive index for theMach-Zender interferometer 16 of 1.46. The wavelength variationdelivered by the laser diode is a consequence of the heating from theincrease in drive current at a rate of roughly 0.7 Angstroms per degreeC. An intensity variation in the output light of the laser also takesplace from this current variation in the drive; however, the presentinvention is insensitive to this intensity variation because it isunnecessary to know the absolute intensity of the light. A Mach-Zenderinterferometer 16 having a 2 cm path difference is utilized for the mostsignificant bit (MSB), with the Mach-Zender interferometers 16 for theother bits requiring larger differences in path length as indicatedbelow in Table I. The least significant bit (LSB) in an eight bit analogto digital converter 10 requires a Mach-Zender interferometer 16 with apath difference of 2.5 m, which is approximately 128 times the 2 cm pathdifference of the most significant bit. A Mach-Zender interferometer 16having a 2.5 m path difference limits the upper frequency which can beconverted to a digital word 24 to something less than 80 MHz, which isthe frequency at which the propagation delay completely prevents opticalinterference of the signal with itself.

                  TABLE I                                                         ______________________________________                                        Parameters for a 0.8 Angstrom Wavelength Variation                            Digital               Intensity                                                                              Upper Interference                             Bit    Path Difference (m)                                                                          Cycles   Frequency (MHz)                                ______________________________________                                        7   MSB    0.02           1      10000                                        6          0.04           2      5100                                         5          0.08           4      2600                                         4          0.16           8      1300                                         3          0.32           16     640                                          2          0.64           32     320                                          1          1.28           64     160                                          0   LSB    2.56           128    80                                           ______________________________________                                    

For an amplitude to optical wavelength converter 12 covering awavelength range that is 100 times larger, namely 8.0 nm, the pathlength differences can be reduced by a factor of 100 and the upperfrequency limit for analog to digital conversion can be set at 4 Ghz foreight-bit resolution.

It will be appreciated by persons skilled in the art that the presentinvention is not limited to what has been shown and describedhereinabove, nor the dimensions of sizes of the physical implementationdescribed immediately above. The scope of invention is limited solely bythe claims which follow.

What is claimed is:
 1. An apparatus for converting an analog signal intoa digital signal comprising:a converter which converts said analogsignal into an optical signal which varies in wavelength in accordancewith an amplitude of said analog signal; a splitter which applies saidoptical signal over a preselected number of light paths; aninterferometer connected to each of said light paths which applies acomplementary sinusoidal transfer function to said optical signal togenerate two complimentary output signals; and, a balanced detectorconnected to each of said interferometers which generates a digital bitin response to said two complimentary output signals, said balanceddetector comprises:first and second photodiodes connected in series in abalanced configuration which generate a resulting current in response tosaid complimentary output signals; and, an amplifier which generatessaid digital bit in response to said resulting current, wherein each ofsaid digital bits are combined to form a parallel digital word.
 2. Amethod for converting an analog signal into a digital signal, comprisingthe steps of:converting said analog signal into an optical signal whichvaries in wavelength in accordance with an amplitude of said analogsignal; splitting said optical signal over a preselected number of lightpaths; applying a complimentary sinusoidal transfer function to saidoptical signal over each of said light paths to generate twocomplimentary output signals; connecting first and second photodiodes inseries in a balanced configuration for generating a resulting current inresponse to said two complimentary output signals; and, generating saiddigital bit in response to said resulting current, wherein each of saiddigital bits are combined to form a parallel digital word.
 3. Anapparatus for converting an analog signal into a digital signalcomprising:a converter which converts said analog signal into an opticalsignal which varies in wavelength in accordance with an amplitude ofsaid analog signal; a splitter which applies said optical signal over apreselected number of light paths; an interferometer connected to eachof said light paths which applies a complementary sinusoidal transferfunction to said optical signal to generate two complimentary outputsignals, where a most significant bit of said digital signal undergoes asingle complementary sinusoidal change for said analog signal; and, adetector connected to each of said interferometers which generates adigital bit in response to said two complimentary output signals,wherein each of said digital bits are combined to form a paralleldigital word.
 4. The apparatus claimed in claim 3, where the second mostsignificant bit of said digital signal undergoes two complementarysinusoidal changes for said analog signal.
 5. The apparatus claimed inclaim 4, where the third most significant bit of said digital signalundergoes four complementary sinusoidal changes for said analog signal;and, each less significant bit of said digital signal each undergosuccessively larger powers of two complimentary sinusoidal changes forsaid analog signal.
 6. A method for converting an analog signal into adigital signal, comprising the steps of:converting said analog signalinto an optical signal which varies in wavelength in accordance with anamplitude of said analog signal; splitting said optical signal over apreselected number of light paths; applying a complimentary sinusoidaltransfer function to said optical signal over each of said light pathsto generate two complimentary output signals; and, determining which ofsaid two complimentary output signals is greater in intensity togenerate a digital bit in response to said two complimentary outputsignals, wherein each of said digital bits are combined to form aparallel digital word.
 7. The method claimed in claim 6, wherein saidstep of applying a complimentary sinusoidal transfer function furthercomprises the step of:applying a single sinusoidal change to determine amost significant bit of said digital signal.
 8. The method claimed inclaim 7, wherein said step of applying a single sinusoidal change todetermine a most significant bit of said digital signal furthercomprises the step of:applying two sinusoidal changes to determine asecond most significant bit of said digital signal.
 9. The methodclaimed in claim 8, wherein said step of applying two sinusoidal changesto determine a second most significant bit of said digital signalfurther comprises the steps of:applying four sinusoidal changes todetermine a third most significant bit of said digital signal; andapplying successively larger powers of two complimentary sinusoidalchanges to determine each less significant bit of said digital signal.